This invention relates to thermochemical removal of metal from metal bodies, commonly called scarfing, and more specifically, to a spot scarfing method capable of producing a fin-free cut, particularly suited for mechanized use. Spot scarfing is a process for scarfing those specific areas of the surface of a workpiece which contain defects, as distinguished from desurfacing the entire surface.
The problems associated with surface conditioning in the steel industry have underscored the need for mechanized spot scarfing of metal bodies, such as steel slabs and blooms. When such a body contains only a few minor defects, scarfing its full surface, i.e. removing a relatively uniform surface layer of metal from the entire work surface, to remove these defects wastes clean, defect-free metal. When the metal body contains many defects, it is common practice to first desurface the entire body, regulating the depth of scarfing cut to remove the majority of the defects, and then to spot scarf the body to remove the remaining, deeper-lying defects. This practice is employed because regulating the depth of the desurfacing cut to remove all, including the deepest, defects would unnecessarily waste good metal. Therefore, spot scarfing, and in particular, mechanized spot scarfing is important for achieving maximum economy in steel conditioning.
One of the major problems associated with spot scarfing metal bodies with conventional scarfing nozzles, either in a mechanized process or by hand scarfing, is the formation of "fins" at the edges of the scarfing pass. A "fin" may be defined as a thin flash or wash of pure or slightly oxidized metal solidly joined to the boundary of a scarfing cut at the surface of the metal workpiece. Such fins must be removed before the workpiece is subsequently rolled, or the fins themselves become undesirable defects. Insofar as spot scarfing is concerned, a fin is formed when molten metal is driven laterally out of the primary reaction zone by the scarfing oxygen stream where the molten metal resolidifies and adheres to the workpiece at the edges of the scarfing cut.
Conventional scarfing processes heretofore used for spot scarfing have employed a wide variety of nozzles. The most common shapes of the oxygen discharge orifice have been either round (such as shown in U.S. Pat. No. 2,309,096 to Bucknam et al), slotted with round ends (such as shown in U.S. Pat No. 2,664,368 to Babcock et al), rectangular (as shown, for example, In U.S. Pat. No. 2,622,048 to Moesinger), or a continuous slot (described in U.S. Pat. Nos. 2,838,431 and 3,231,431 to Allmang et al). These types of nozzles all produce fins from the primary reaction zone. In an effort to minimize the problem of fin formation, it has been a practice to use the above nozzles in conjunction with jets of air, water or the like which are directed at the incipiently forming fins so as to push the fin-forming molten metal back into the reaction zone. Thus, for example, Japanese Utility Model Application Publication No. 31066/1972 discloses blowing a stream of high pressure air or oxygen for this purpose, while Japanese Patent Application Publication No. 14126/1971 discloses the use of a water jet for removing residual slag from the edges of a scarfing cut. This procedure has met with some degree of success in that it is possible to achieve a relatively shallow fin-free cut with a rectangular or continuous slot nozzle operating within a narrow range of scarfing oxygen pressure and scarfing speeds. However, the control of process variables to minimize fin formation by this technique becomes so critical, the scarfing reaction so unstable, and the depth of the cut so shallow, that such a scarfing operation is commercially impractical.
Other methods have been employed to compensate for the basic inability of conventional scarfing nozzles to produce a fin-free cut. These include directing a single oxygen nozzle at an angle relative to the scarfing path as disclosed, for example, In U.S. Pat. No. 2,125,179 to Doyle, and inclining two or more nozzles towards each other so that their scarfing oxygen streams intersect essentially on the center line of the desired cut path, a technique shown in Doyle and in U.S. Pat. No. 2,157,095 to Bucknam. Using a single inclined nozzle as described in Doyle avoids fin formation on the near side of a cut but aggravates fin formation on the far side, and results in a non-symmetrical cut cross-section. Two mutually inclined nozzles as described above can be employed successfully to make a fin-free cut, but the cut contour is characterized by a deep groove along the path of intersection of the oxygen streams and results in an undesirable surface contour for spot scarfing.